A question in Time Series:
We have Augmented Dickey-Fuller (ADF) test to test the stationarity of time series. But it seems that this test behaves weirdly. Even for time series which obviously look non-stationary just by looking at it, this test says that it is stationary. For example, check the result of this test for the "AirPassengers" dataset, it will give you a p-value of less than 0.01 (< 0.05), which suggests that the series is stationary. This seems to be misleading.
Some resources say that this test checks if the characteristic equation of the series has a unit root. Stationary series have roots > 1.
Can someone help me with this? What is correct here?